And, the Echo is ever so weak.You did not mention the separate small speaker cab used for the echo. Do you have it? You must have a separate speaker to hear echo (reverb) from this amp.
You did not mention the separate small speaker cab used for the echo. Do you have it? You must have a separate speaker to hear echo (reverb) from this amp.
I don't trust that hand drawn schematic. Output stage looks whacky.
I think the OT is drawn in two pieces. If you slide the secondary up-over next to the primary it looks conventional.
If you slide the secondary up-over next to the primary it looks conventional.Yes, I considered that. But the 68µF between the secondary and the RCA speaker jack still looks whacky.
the 68µF between the secondary and the RCA speaker jack still looks whacky.
Or strap it. Signal will pass. I suspect it is about the specific speaker they used: great mid-high output but didn't handle bass gracefully. Since reverb for effect is mostly midrange, bass-chop is appropriate, but strapping will tell you signal gets through.
Please explain strapping.Put a jumper wire across the cap.
I would check out the voltage divider resistors that connect to pin=9, R56, R57, and R58, are the correct values and are up to spec.
The heck with learning a new graphics editor. I can do it in CAD
Should I just go ahead, bite the bullet, and replace all caps from V5, V6, V8a, through V7c?No. Divide and conquer. Make the oscillator V7c work and the rest of the trem circuit will probably work.
No. Divide and conquer. Make the oscillator V7c work and the rest of the trem circuit will probably work.
Remove the lamp limiter if still connected and plug the amp straight into the wall. V7c needs full B+ to operate. Check voltages on plate, grid, and cathode of V7c. If oscillating properly the plate voltage will be fluctuating at the trem frequency and will just appear erratic on most dmms. A scope or analog meter would be very nice to monitor the plate. The trem signal on the plate should be a BIG amplitude low frequency AC sine wave. You can view this signal with a scope. Or, your dmm might be able to measure it. My Fluke 87 can measure the RMS voltage as well as read the frequency of most tube trem signals.
If your V7c is not oscillating, swap V7 and V8 first, or subsitute a good tube for V7 if you have a spare. If still no go, change C20, C21, and C22 (.047µF is fine). Next check resistors in V7c circuit and measure resistance to ground for the grid and cathode.
Hopefully the trem will be working at this point. Let us know the results of these few checks.
V7c needs full B+ to operate. Check voltages on plate, grid, and cathode of V7c. If oscillating properly the plate voltage will be fluctuating at the trem frequency and will just appear erratic on most dmms. A scope or analog meter would be very nice to monitor the plate. The trem signal on the plate should be a BIG amplitude low frequency AC sine wave. You can view this signal with a scope. Or, your dmm might be able to measure it. My Fluke 87 can measure the RMS voltage as well as read the frequency of most tube trem signals.
If your V7c is not oscillating, swap V7 and V8 first, or subsitute a good tube for V7 if you have a spare. If still no go, change C20, C21, and C22 (.047µF is fine). Next check resistors in V7c circuit and measure resistance to ground for the grid and cathode.
I'm hoping someone will tell me why a triode won't oscillate and another like it will.Because one tube is a little stronger than the other.
the tremolo needs to charge up at the highest speed ..like off on mine and then turn the speed down .
With the speed pot at minimum, it took a very long time for the needle to begin to deflect and oscillate, after switching the Vibrato on. However, with the speed pot at maximum, I got instant oscillation.I did a simple mod on my recent J-12 Jet build that will take care of that slow startup issue. I highly recommend you do this mod too. It's easy to do or undo. Look at this schematic and read the conversion notes...
I tried connecting the footswitch to the pot wiper. Loud pop.I just checked my J-12. It pops too! But my pop is not objectionable with the little 6" speaker. It is too loud when using a big speaker though. Sorry 'bout that. :embarrassed:
I have a solution for the slow trem startup. I've tested this on my J12 and it works fine. Fast startup and no pops.
Refer to your schematic...
1. Connect R30 straight to ground.
2. Remove the ground connection from the bottom of the speed pot.
3. Connect the bottom of the speed pot to V7c pin 3 (cathode).
4. Connect the footswitch between ground and the junction of C21 and C22.
This is basically what Fender did in the 6Gxx series. Gives fast startup and no pop.
Jack does your amp have this wire ????
The RC circuit is a high pass filter. It removes thumping which can sometimes be heard in tremolo outputs.
Check out the VOX AC-30 and AC-15 schematics and you'll see something very similar being done.
xm52 -
Here are a couple links to the old Gibsons that I have:
'55 and later (50's, that is):
The '55 has the high pass filter that you spoke of, along the lines of the Ampeg and Vox's. The '54 and earlier have none that I can tell. It has a very noticeable thumping, when turned on. I don't care. It is a wonderful sounding amp, and very quiet. This was one of those slightly cannibalized/abandoned amps. Decoupling caps bad, and all tubes missing. Replaced and it is a keeper.
The '55 needed some of the tubes and the chassis reinstalled. For some reason, the threads were stripped, for the two mouning screws. I made a quick fix on it by just putting nuts and new 10-32 x 1-1/4" screws in place of the missing ones. I haven't had that one back for several months, to make the permanent repairs to the stripped holes.
Is this the very subtle thumping that I'm hearing in this amp, with volumes turned down and Vibrato turned on? My early GA-40's do this, also. One is rather noticeable. I bought .1uf caps for C13,14,15, and 16, but have not put them in. On this amp, you can't hardly hear it. You can, for sure, in the 40's.
Any thoughts on the extra stages that the oscillator feeds?
Have a Blessed and Safe Christmas.
You too Jack! Take a break with your family and church, you deserve it.
Did I ever say I really love your bottom line personal signature?
As a greybeard, it is unlikely you saw those words that way as a kid.
While it is something Orwell *might* have said, there is no record of that phrase before Richard Grenier (http://en.wikipedia.org/wiki/Richard_Grenier) used it in 1993.
When you use a jumper and short a tube pin to ground, that's just grounded on the chassis? Are there specific pins you DO NOT ground 'cuz they have HT volts? And, this then would isolate the noise culprit when a pin shorted makes the amp go quiet? Do you use this method on Rs and Cs also?Using a ground strap in the signal chain can be a very useful troubleshooting tool. But doing so requires an understanding of schematics and how circuits operate. Lacking that understanding and using a ground strap is an invitation to disaster.
When you use a jumper and short a tube pin to ground, that's just grounded on the chassis? Are there specific pins you DO NOT ground 'cuz they have HT volts? And, this then would isolate the noise culprit when a pin shorted makes the amp go quiet? Do you use this method on Rs and Cs also?
I'm going to lift all from V7-pin9, connect it to signal ground with a 1M resistor, and see what it sounds like. And then, short jumper this point to signal ground, as my long jumper picks up way too much noise.
I'm hoping I'll be troubleshooting the input jacks and the resistors/wiring, because the grid grounding was successful in eliminating the hiss and maybe some hum. Again, the hum is not really bad. Just obvious at full volume with nothing attached. But, it all goes away, back at the junction of C2 and the Volume pot. This lets me know that the subtle hum may be something that can be made to go away.
I'm hoping that whatever rids the hiss on Channel 1 will be the same solution to rid Channel 2 of it's big hiss (again, at full volume, no connections).